1. Field of the Invention
This invention relates to an improved method for the production of pulps of high filler content in which filler is loaded in the lumens of the cellulose fibres and to novel pulps produced using such method.
More specifically, the present invention relates to a novel method of producing paper containing high concentrations of calcium carbonate filler and to novel paper produced by the method.
2. Description of the Prior Art
The increasing use of calcium carbonate as a filler in fine papers has been a major trend in recent years. The resultant alkaline sheets are brighter, stronger, have superior printability and are more permanent than sheets made under acidic conditions. In addition, the use of calcium carbonate is a means of reducing the furnish costs by substituting fibre with less expensive filler. With these incentives, many papermakers strive to raise the filler content as high as possible. However, as filler content is increased, paper strength is reduced resulting in poor papermachine runnability. Fillers contribute nothing to paper strength themselves and lower the concentration of load-bearing fibres. In addition, filler particles accumulate on exterior fibre surfaces reducing paper strength by interfering with inter-fibre bonding.
Green, Fox and Scallan (U.S. Pat. No. 4,510,020) describe one approach to improving the strength of papers containing fillers. They disclose a method of loading the filler within the fibre lumens where it does not interfere with fibre--fibre bonding. Thus, the potential is there for greater filler contents in the paper and better paper machine runnability. The basic process of lumen loading involves an impregnation step in which the pulp is agitated in a concentrated suspension of filler to allow the filler particles to enter the lumens via pit apertures. If attractive forces between the filler particles and the fibre surfaces exist, the filler bonds to both exterior and the lumen surfaces of the fibres. In a subsequent step the particles on the exterior surfaces of the fibres are removed by washing the pulp. For the most part, the disclosure is focused on the use of titanium dioxide fillers which proved to be very suitable for lumen loading.
Application of the lumen-loading principle to calcium carbonate fillers was mentioned as possible in U.S. Pat. No. 4,510,020 but no examples were given. Okayama et al. Japan Tappi, 43(5), 495, (1989) found that calcium carbonate, in the size range of commercial fillers, generally loaded to levels of less than 0.08 g/g of fibre; a much lower loading level than titanium dioxide under comparable conditions. A value of 0.15 g/g was obtained on a calcium carbonate of 0.1 .mu.m diameter--well below the size of commercially available fillers of practical and economic importance in papermaking.
Retention aids have been proposed to promote lumen loading of fillers. Middleton and Scallan, J. Pulp Paper Sci., 15(6), 229 (1989) have described the use of a cationic polyacrylamide at pH 4 to increase lumen loading using titanium dioxide. Miller and Paliwal, J. Pulp Paper Sci., 11(3), 84, (1985) have described the use of polyethylenimine to increase the levels of lumen loading using titanium dioxide and clay fillers. A process for lumen loading calcium carbonate using polyethyleneimine is described by Chang et al, Taga Proceedings 1997 (TAGA), Session: Experimental Analysis of Printing, p639-657, 1997. Using a precipitated calcium carbonate, Chang et al reported loading levels of only 1-5% with a brief mention of a maximum level of 10.8% being achieved using 8% polymer addition and mixing being carried out at a pH of 13. These conditions of a very high polymer addition and a very high pH would be a severe barrier to practical implementation in a mill. Another method for lumen loading calcium carbonate is reported by Hockman and Sohara, International Publication Number WO 98/35095. In this method filler and fibre are mixed together so as to effect lumen-loading. This is followed by the addition of a flocculating agent to prevent the filler diffusing outside the lumens. Levels of loading of up to 10% were claimed.
There have been other approaches to producing pulps containing calcium carbonate formed by "in situ" precipitation. Allan et al. U.S. Pat. Nos. 5,096,539 and 5,275,699, for example, saturate fibres with calcium chloride solution and then add sodium carbonate solution. However, in addition to producing calcium carbonate, the process leaves sodium chloride as a by-product which is considered detrimental in any commercial application. In an attempt to avoid such a by-product, Klungness et al. U.S. Pat. No. 5,223,090 impregnate fibres with calcium hydroxide solution and then apply an atmosphere of carbon dioxide to precipitate calcium carbonate. Both precipitation procedures produce calcium carbonate in various locations in a pulp. Klungness et al reported that the filler actually in the lumen was less than 0.06 g filler/g fibre.
For both "in situ" precipitation techniques, the claims in terms of benefits for the paper sheet are similar to those of lumen loading. These benefits are improved retention of filler during sheet formation and superior sheet strength over conventionally-filled sheets, i.e., where all the filler is retained on the outer surfaces of the fibres. The two precipitation techniques have common disadvantages. The first is the difficulty of obtaining an optimum size distribution of the filler for maximum optical properties. In contrast, commercial precipitated calcium carbonate is manufactured to specific particle sizes to produce optimum light-scattering characteristics. The second is that much filler is not within the lumen but external to the fibre i.e., where it causes a loss of sheet strength. In addition, "in-situ" procedures call for marked deviations from common papermaking practices.
At present, two classes of calcium carbonate fillers are commercially available. The first is a "ground" filler prepared by mechanically grinding naturally occurring deposits such as chalk or limestone. The other class is a "precipitated" filler prepared from a solution by addition of a reactant bringing about a precipitation of calcium carbonate. Within the two classes there are various grades based on particle size and shape. However, a chemical difference between the two is that the "ground" filler usually contains an adsorbed dispersant rendering its particles with a negative electrical charge while the "precipitated" filler usually has no such additive and its particles retain their natural weakly positive charge. Although the terms ground and precipitated are used in this specification it is the aspect of the electrical charge of the filler particles to which we are referring rather than the method of preparation of the filler.